1. Field of the Invention
This invention relates to an optical disc on which optically modulated video, audio and other information signals are recorded and a method of manufacturing the optical disc.
2. Description of the Prior Art
Some optical information recording and reproducing systems have recently been developed. The optical information recording and reproducing system can record video, audio and other information signals, as optically modulated signals, onto an optical recording disc (optical disc, hereafter) as shown in FIG. 1 and reproduces the recorded signals from the optical disc by using a laser beam.
FIGS. 2 shows a partial sectional view, taken along the line Y-Z in FIG. 1, of a typical conventional single-side recordable optical disc. A disc substrate 1 is made by compression-molding or injection-molding transparent glass or resin material. A laser beam guiding groove 15 is formed on one surface of the disc substrate 1 in a spiral shape to guide the laser beam during the recording or reproducing operation. The number of turns of the spiral groove 15 is 15000-24000. This groove 15 may be omitted. A recording medium layer 2 generally made of tellurium alloy is deposited by vapor deposition, for example, on the surface provided with the spiral laser beam guiding groove 15 of the substrate 1. The optically modulated signals are recorded on this recording medium layer 2. The recording medium layer 2 is generally made of a material such as bismuth, molybdenum, tellurium, cobalt, nickel, arsenic, zinc, and tin. A protective resin layer 4 is adhered to the recording medium layer 2 with a adhering layer 3 for protecting the recording medium layer 2 from dust and the like.
FIG. 3 shows a partial sectional view, taken along the line X-Y in FIG. 1, of a typical conventional double-sides recordable optical disc, which is the so-called air sandiwch type optical disc. Two disc substrates 1a, 1b are formed by the compression-molding or the injection-molding, each substrate having a laser beam guiding groove 15 spirally formed on one surface thereof. Recording medium layers 2a 2b are formed by such as vapor deposition on the groove-formed surfaces of the disc substrates 1a, 1b, respectively. The recording medium layers 2a, 2b are generally made of tellurium alloy. The two disc substrates with recording medium layers are fixed at the recording medium layer sides with spacer rings 17 which are provided at outer and inner peripheries of the disc, so that an air layer 16 is provided between the recording medium layers 2a, 2b.
When signals are recorded onto or reproduced from the optical discs as shown in FIGS. 2 and 3 the laser beam is incident upon the recording medium layer from the disc substrate side of the optical disc. Referring to FIG. 1 (or FIG. 2), the laser beam 12 radiated from a laser 9, when recording, is passed through an optical system 10 and converged by an objective lens 14 to focus on the recording medium layer 2 through the transparent disc substrate 1. The laser beam 12 is focused to form a small spot of about 1 .mu.m in the diameter at the surface of the recording medium layer 2. A part of the laser beam reflected from the recording medium layer 2, when reproducing, passes the beam on same path back through the objective lens 14 and to the optical system 10, and is further lead to a photodetector 11 which converts the reflected laser beam to an electrical signal 13. In this case, in the vicinity of the surface facing to the objective lens 14 of the disc substrate 1, the laser beam which is not adequately converged there is less intercepted by foreign matter existing there. However, in the vicinity of the recording medium layer 2, the laser beam, which is adequately focused to be a small spot of about 1 .mu.m in diameter there, is intercepted by a foreign matter of about 1 .mu.m in size existing there. In other words, the recording or reproducing error rate (which is defined as the possibility of losing an information bit with respect to the number of recording or reproducing information bits) is largely affected by the purity of the transparent disc substrate in the vicinity of the recording medium layer.
Accordingly, the conventional optical disc requires a very high quality transparent disc substrate which contains quite a few foreign matters and is optically uniform throughout the substrate. Actually, it is very difficult to select and mold a material suitable to satisfy the above requirement.